In recent years, in super high-speed communication beyond 100 Gbit/second, a communication technology by polarized wave orthogonal multiplexing multiple value digital signal modulation method (DP-QPSK: Dual Polarization Differential Quadrature Phase Shift Keying) excelling in wavelength utilization efficiency, reception characteristics and dispersion compensation ability is noted. For a receiver in DP-QPSK system, the function of polarized wave separation of an optical signal into a TE (Transverse Electric) optical signal and a TM (Transverse Magnetic) optical signal, and the function of 90 degree optical hybrid for extracting the phase information out of these optical signals which have been performed with the polarized wave separation are necessary. This phase information is the four value phase information on an I-Q plane, composed of Ip and In which have π phase difference each other, and also Qp and Qn which have π/2 phase delay to Ip and In respectively.
A planar optical wave circuit using optical waveguide technology is considered superior for a device which realizes the receiver function of such DP-QPSK system, and its development is advanced in recent years. The optical waveguide technology is the technology which sets up an optical waveguide with the various shapes on a substrate by the same fine processing technology as a semiconductor integrated circuit manufacturing process, and is suitable for integration and mass production.
For example, as an optical waveguide device integrating the polarized wave separation function mentioned above and the 90 degree optical hybrid function into a planar optical circuit, an optical wave circuit structure shown in FIG. 8 is disclosed in non-patent literature 1. FIG. 9 indicates the composition of a planar optical circuit of the TE optical signal side out of the part which performs the 90 degree optical hybrid function, as a schematic diagram.
The optical wave circuit shown in FIG. 9 composes an interferometer which is generally called a coherent mixer. In FIG. 9, the inputted TE optical signal and local oscillator light are branched by optical branching devices 10 and 11, respectively. A Y branch structure type optical branch device which is the most basic optical branching device is usually employed as the optical branching devices 10 and 11. The reason is that Y branch structure type optical branching devices basically have no wavelength dependence on the optical branching ratio and are relatively tolerant of the disturbance on manufacturing because of employing the simple symmetrical structure in which the inputted light is split from one waveguide into two branches and outputted. The optical waveguide arms 12-15 compose an interferometer, and the arms 12-14 are the same in the optical path length while the optical path length of the arm 15 is longer than other arms by equivalent of ¼ of the wavelength of optical wave propagating in the optical waveguide, so that the 90 degree phase difference is given. That is, the relation between the optical path length difference dL of two arms and the phase difference d φ of the lights traveling through those arms is expressed by the formula (1), wherein the wavelength is λ and the effective refractive index of the optical waveguide is n.dφ=2π·n·dL/λ  (1)
According to the formula (1), the optical path length difference dL corresponding to the 90 degree (π/2 radian) phase difference is represented by the formula (2).dL=λ/4n  (2)
Accordingly, by setting up the optical path length difference to the optical waveguide arms as mentioned above, the four-value-phase-information on the I-Q plane is outputted from the optical couplers 16 and 17, and the above-mentioned 90 degree optical hybrid function is realized.